Electronic countermeasures deception systems are, in certain instances, required to generate an array of false targets in angle and range against the threat TWS type radar. The angle program can be accomplished by phase locking an angle clock to the incoming scan rate. The clock has a high basic frequency that is counted down, and the various counts then form the angular positions. However, some aspects of positioning the targets in range are not as easily accomplished.
The video circuitry of such a deception system sees a pulse train at a nominal 1 KHZ rate derived from the RF pulses coming from the threat radar. The repetition rate is determined by the electromagnetic propagation velocity to give the radar an unambiguous range of approximately 90 miles. When the illuminated aircraft is relatively close to the site, typically less than 30 miles, the radar usually changes its mode to double the PRF. This gives a higher data rate to their servos so that the site can launch its surface to air missiles. The time from the trigger point of their transmission till the returning echo is received determines the range as far as the radar is concerned. The way the ECM creates the illusion of other targets in range is by generating pulses of the same RF frequency before and after the radar pulse, and of course having the appropriate angle modulation. For instance, using the approximation that a foot is transversed in a nanosecond at the speed of light, then if the ECM generates a pulse eight microseconds after the radar pulse impinged on the aircraft, the radar will think it sees a second craft four thousand feet behind the plane.
In order to generate these pulses to follow the threat radar pulses, it is possible to simply use timing multivibrators. If the timing is long enough then the position will be close to the next pulse that is due from the radar, so the radar site operators will think that this ECM pulse is a target just in front of the true range. However this method of generating leading targets is not practical since although the PRF is known approximately, there can be variations within this region. If it is desired to always put a pulse out just a few microseconds before the expected radar pulse, then the fixed timing multi vibrators are grossly inadequate. For this reason an analog-digital hybrid range clock that phase locked to the input pulse train has been used. The voltage controlled oscillator of this system runs at a much higher rate then the input and is counted down. Early and late gates provide the error signals for constant phase and frequency correction. The loop inertia is such that false triggers (usually in the early gate period) generally do not cause the acquisition on the original pulse train to be broken, and the stable point of the loop is also the error level null point so that the tracker can even hold through periods of missing pulses. The various counts from this clock then, serve as the trigger points for the range positions. Therefore any range position is available through the period provided that it is a multiple of the oscillator period from the input trigger point. For example, the clock period can be taken as four microseconds, so that the count just before the true range clock can be used to seemingly put a target two thousand feet in front of the aircraft independent of the exact radar PRF. If the pulse rate is different, then the range clock will just have a different number of counts to fill the interpulse period, and the first preceeding count will still lead by four microseconds.
Therefore, for the low PRF case, the above mentioned use of a range clock provides an excellent method of generating the false target in range program. The region of lock-up can be set to cover all the low PRF possibilities. Now if the radar switched to high PRF mode by exactly doubling its frequency, then the ECM logic program could determine the mid count and place the same array of targets around it as there were around the low PRF pulse, at least it could if it were ensured that there were an even number of counts in the interpulse period. Therefore it would not be much of a problem to handle the high PRF mode.
This high PRF mode capability, however, is complicated by the fact that when the radar switches to this mode the average pulse repetition rate does exactly double, but the high pulses are not placed midway between the low pulses. This is because the radar has an MTI capability in order to get a measurement of the aircraft velocity. By staggering the pulses, typically the low period is divided by a thirteen units to twelve units ratio, they eliminate much of the beating effect between the doppler shift and the pulse rate. Since delay lines or their equivalent are used to accurately measure the differences between successive pulses, the interval from the low pulse to the high pulse is very precise for each site, although the ECM system may not know exactly where it will appear in the period.
One approach toward solving this problem has been to use two identical range clocks, one of which would lock to the low PRF, and the other to lock to the high PRF pulses, if they were present. This approach is unsatisfactory, however, because if the basic oscillators of the two clocks differed in frequency by just a small fraction of one percent, then there would be a different number of counts in each set of counters that measured the interpulse period.
Accordingly, it appears that the basic oscillator frequency of the primary range clock must be employed to handle the high pulses. However, it was not known how many counts in the interval from the low to the high pulse would exist. This can be mechanized of course by the same count determination technique used in the primary range clock. The problem is that in general there would not be an exact interger of counts in this interval. If this clocking square wave were used to position the false targets around the high pulse, then substantial error may be introduced, with a maximum possible error being one half the clock period.
An alternative approach has been to use a bank of four gang tuned voltage controlled multi vibrators. If the clock period is four microseconds, then each multi vibrator would have a timing cycle of from two to six microseconds, with a minimum of two microseconds recovery time. The first multi vibrator is triggered off every other positive edge of the clock waveform giving 360 degree operation. The output of this circuitry is a reconstructed square wave of variable phase with respect to the input.
The bank of four multi vibrators approach, however, is difficult to implement successfully. In the first place it barely covers 360 degrees operating range. In the second place two of the multis trigger off the opposite polarity edge of the clock waveform which means that the design of all four will not be identical. Overall it is very difficult to get a symmetric looking square wave from the circuit. With the same control voltage applied to them, they did not have equal timing cycles. If they are made equal at one voltage, it does not ensure that they would track properly. Consequently, although ganged multi vibrators can be made to work, the alignment procedure is very involved, and the number of adjusting potentiometer prohibitive.